Derangements in the G1 cell cycle machinery, including overexpression of cyclin D1 and suppression of the cyclin-dependent kinase inhibitors p21(cip1) and p27(kip1), appear to be important in the pathogenesis of breast cancer. As key mediators of cytoskeletal organization and integrin-mediated signaling, the Rho family GTPases are precisely situated to play a key role in regulating these proteins. The goal of the present studies is to evaluate whether inactivation of specific Rho GTPases and their downstream targets can normalize aberrant levels of cyclin Dl, p21(cip1) and p27(kip1) and inhibit autonomous proliferation of breast cancer cells. Specifically, aim 1 is to clarify the role of Rho proteins in adhesion-dependent G1 cell cycle progression in breast epithelial cells. Aim 2 evaluates the ability of dominant negative forms of Rho GTPases to correct aberrant expression levels of cyclin D1, p21(cip1), and p27(kip1) in tumorigenic breast cancer cells (including primary breast cancer cells). Aim 3 analyzes the signaling pathways through which Rho GTPases influence G1 cell cycle progression. Aim 4 addresses whether inhibition of these pathways can affect tumorigenicity. The P.I., Dr. Catherine Welsh, is committed to pursuing long-term goals of enhancing the understanding of cell cycle deregulation in breast cancer pathogenesis as an independent physician-scientist, an effort which should lay the groundwork for identifying future therapeutic targets. The current proposal provides her with the mechanism to achieve these broad goals by meeting her more immediate needs. First it allows the extension of promising preliminary data to the understanding of breast cancer proliferation under the guidance of Dr. Kermit Carraway who has made major contributions in breast epithelial and tumor cell biology. It also allows the continuation of a highly effective and productive relationship with Dr. Richard Assoian who will continue to provide expert guidance on cell cycle, adhesion, and Rho GTPases. The multidisciplinary expertise and insight provided by these mentors are a central component of the career development plan. The University of Miami is committed to supporting Dr. Welsh's career development and to this end, has provided her with ample protected time and laboratory facilities to meet her goals. Thus, the proposed award would provide an ideal means to extend a promising line of work into understanding breast cancer growth while simultaneously facilitating Dr. Welsh's progression to the status of independent investigator.